The present invention relates to a method of analyzing a sample for the presence of a nucleic acid. More particularly, the present invention is directed to an automated method for detecting and reporting the presence of a predetermined nucleic acid in a sample using polymerase chain reaction and a fluorescent detecting entity.
Amplification of DNA by polymerase chain reaction (PCR) is a technique fundamental to molecular biology. Nucleic acid analysis by PCR requires sample preparation, amplification, and product analysis. Although these steps are usually performed sequentially, amplification and analysis can occur simultaneously. DNA dyes or fluorescent probes can be added to the PCR mixture before amplification and used to analyze PCR products during amplification. Sample analysis occurs concurrently with amplification in the same tube within the same instrument. This combined approach decreases sample handling, saves time, and greatly reduces the risk of product contamination for subsequent reactions, as there is no need to remove the samples from their closed containers for further analysis. The concept of combining amplification with product analysis has become known as xe2x80x9creal timexe2x80x9d PCR. See, for example, WO/9746707A2, WO/9746712A2, WO/9746714A1, all published Dec. 11, 1997, incorporated herein by reference.
Monitoring fluorescence each cycle of PCR initially involved the use of ethidium bromide. Higuchi R, G Dollinger, P S Walsh and R. Griffith, Simultaneous amplification and detection of specific DNA sequences, Bio/Technology 10:413-417, 1992; Higuchi R, C Fockler G Dollinger and R Watson, Kinetic PCR analysis: real time monitoring of DNA amplification reactions, Bio/Technology 11:1026-1030, 1993. In that system fluorescence is measured once per cycle as a relative measure of product concentration. Ethidium bromide detects double stranded DNA; if template is present fluorescence intensity increases with temperature cycling. Furthermore, the cycle number where an increase in fluorescence is first detected increases inversely proportionally to the log of the initial template concentration. Other fluorescent systems have been developed that are capable of providing additional data concerning the nucleic acid concentration and sequence.
While PCR is an invaluable molecular biology tool, the practical implementation of real time PCR techniques has lagged behind the conceptual promise. Currently available instrumentation does not actually analyze data during PCR; it simply acquires the data for later analysis. After PCR has been completed, multiple manual steps are necessary to analyze the acquired data, and human judgment is typically required to provide the analysis result. What is needed is a system for automating data acquisition and analysis so that no user intervention is required for reporting the analytical results. Thus, when the temperature cycling in a polymerase chain reaction amplification is complete, the system software is automatically triggered and the results, for example, the presence or absence of a given pathogen, is immediately displayed on screen. Algorithms for detection, quantification, and genotyping are needed. Moreover, initiation of the analysis algorithm can be implemented prior to completion of temperature cycling. Data processing can occur during amplification and concomitant analysis results can be used to modify temperature cycling and to acquire additional data during the latter stages of the amplification procedure to optimize amplification protocol and data quality.
A major problem in automating PCR data analysis is identification of baseline fluorescence. Background fluorescence varies from reaction to reaction. Moreover, baseline drift, wherein fluorescence increases or decreases without relation to amplification of nucleic acids in the sample, is a common occurrence. Prior attempts to automate amplification data analysis involved setting the baseline fluorescence as that measured at one or more predetermined early cycle numbers. This technique accounts for the variation in background fluorescence, but it does not compensate for baseline drift. Without compensation for baseline drift, automated amplification data analysis can easily provide both false negative and false positive results.
Thus, one aspect of the present invention is directed to a method of determining the presence of a nucleic acid in a sample by using a fluorescent entity capable of detecting the nucleic acid and amplifying the nucleic acid in the presence of the fluorescent entity. A baseline fluorescence region is determined by analyzing the fluorescence measurements of a number of amplification cycles, and the fluorescence measurements during specific amplification cycles are compared to the baseline fluorescence region to determine the presence or absence of the nucleic acid. In a preferred embodiment, the baseline fluorescence region is determined by calculating the slope of a fluorescence intensity verses amplification cycle plot at each of the amplification cycles and choosing the fluorescence measurement of the amplification cycle with the slope having an absolute value closest to zero. Preferably, the baseline fluorescence region is generated as the square root of the mean square error.
In another embodiment, the baseline fluorescence region is determined and the fluorescence values are compared thereto after each amplification cycle. Thus, the presence of the nucleic acid sequence can be determined more quickly in samples containing higher copy number. Furthermore, the remaining cycles may be used to acquire other information concerning the nucleic acid sample, such as initial copy number and allelic data.
In an additional embodiment, the process is automated so that a user can prepare a biological sample and simply place it in a thermal cycler having a sensor for reporting fluorescence values as a function of cycle number and a processor programmed with an algorithm capable of processing the values and reporting a positive or negative result.
Additional features of the present invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.